1. Field of the Invention
This invention relates to a process and apparatus for generating high velocity particulate containing fluid jets which are suitable for making notches in blast holes to control fracture formation during the detonation of explosives.
2. Description of the Prior Art
Blasting with explosives is utilized in mining, tunneling, excavation, demolishing operations, and the like, to remove hard, generally impenetrable materials such as ice, rock, minerals, concrete, and the like. In blasting operations, blast holes are drilled into the hard material, generally with a percussive tool or a drill, and the holes are filled with explosives. If a large volume of material is to be removed, a series of blast holes must be drilled in a prescribed pattern to control the pattern of explosion and material removal. Proper spacing and arrangement of blast holes depends upon the properties of the hard material being removed and the amount and type of explosives used.
It is highly desirable to control fracture formation during detonation of explosives so as to control the explosion and removal pattern by linking the blast holes. One common method of controlling fracture formation is to make wedge-shaped notches along a blast hole in the direction of the desired fracture formation. Notching blast holes in hard materials, such as rock, minerals and concrete is, however, very difficult. Carbide or diamond studded cutting wheel tools, saws, and drills have been devised for notching blast holes, but these tools have recognized limitations, such as rapid wear of cutting edges, expense to manufacture and operate, slow, noisy, dusty and fatiguing operation, and excessive fragility for use in most blasting environments.
High velocity water jets generated at pressures of up to 60,000 psi are used industrially to cut various materials, such as paper products, leather, polymers, plastics, textiles and asbestos products. Utilization of high velocity water jets for cutting operations is gaining popularity because of its many inherent advantages, including absence of tool contact and wear, heat and dust generation, and high speed and quality of cuts. U.S. Pat. No. 4,478,368, which is incorporated herein by reference in its entirety, describes high pressure water jet apparatus, applications and technology. Since high velocity water jets can be generated utilizing relatively small nozzles, the water jet apparatus can be inserted directly into a blast hole for notching the rock. In general, however, the application of high velocity water jets to cut hard materials such as rock and concrete has been unsatisfactory, since the water jets tend to cause spalling and fracturing of hard materials, rather than cutting the material cleanly.
Abrasive particles propelled by compressed air have been used to cut many hard materials. This method can be quite effective when the abrasive particles are accelerated to high velocity and ejected through a suitable nozzle. However, the difficulty in containing the particles and dust during cutting operations prohibits its use in large scale material cutting. Currently, air-propelled abrasive powders are used for deburring metals and for surface preparation of materials where a hood or an enclosure can be employed to contain the dust. A wide variety of abrasive powders, such as silicon carbide, aluminum oxide, garnet, glass beads and silica sand are used for such applications.
The combination of solid particulates with a high pressure fluid jet has been utilized for several purposes. For example, U.S. Pat. No. 2,810,396 teaches solid particles in an air or steam injector as an attrition impact pulverizer; U.S. Pat. No. 3,424,386 teaches mixing of granular solids with a liquid for use in sandblasting; U.S. Pat. Nos. 3,972,150 and 3,994,097 teach water jets having particulate abrasives for cleaning with water pressures under 5000 psi; U.S. Pat. No. 4,080,762 teaches a fluid and abrasive jet for paint removal with fluid pressures up to 30,000 psi; and U.S. Pat. No. 4,125,969 teaches a wet abrasion blast cleaning apparatus and method utilizing soluble abrasive materials. U.S. Pat. No. 4,449,332 teaches a nozzle holder for dispensing a water jet containing particulate abrasive material which may be used for cutting or cleaning applications. The nozzle assembly is capable of withstanding high liquid pressures of between about 10,000 to about 50,000 psi.
U.S. Pat. No. 4,478,368 teaches a high velocity particulate containing fluid jet apparatus and process providing improved fluid jet quality by utilizing multiple fluid jets and flow shaping construction. This patent also teaches the supply of solid particulates in a foam for mixture with the fluid jet stream to minimize energy loss of the fluid jet stream and provide better control of the introduction of solid particulates into the fluid stream. Very hard materials, such as concrete, rock, glass and metals, may be cut using fluid jets containing abrasive particulates which have been generated at moderate fluid pressures and at high fluid pressures of up to 60,000 psi. Gene G. Yie, "Cutting Hard Rock with Abrasive-Entrained Waterjet at Moderate Pressures", paper presented at 2d U.S. WaterJet Symposium, Rolla, Mo., May 26, 1983, for example, described that glass can be cut into complicated shapes with abrasive fluid jets when very hard abrasives, such as garnets, are used. Fluid jets containing abrasive particulates may be utilized to make many different types of cuts. The kerf produced by a suitable abrasive water jet nozzle may be as narrow as less than 0.05 inch or as wide as more than 1.0 inch.
In these types of particulate containing fluid jet generators, the factor which determines the cutting capabilities of the abrasive fluid jet is the efficiency of the nozzle assembly in accelerating the particulates in the fluid jet for cutting applications. It is desirable that the velocity of the abrasive fluid jet as it exits the nozzle is as high as possible, and that all particulates introduced be accelerated to a very high speed. It is preferred, in these types of abrasive fluid jet generators, that all fluid and particulate chamber walls have smooth surfaces to minimize fluid turbulence. Mixing of abrasive particulates into a highly pressurized, coherent fluid jet is very difficult to achieve.